Severe pain before periods, scanty menses flow. What is the cause?

hi i m 20 yr female virgin , i have a very sever pain b4 the day of period and on the very 1st day of period because of that i have a scanty amount of period and which only last for 3 1/2 days and pleez tell me what is the problem it feels like i will have heavy periods when i have the shooting painbt i have less flow earlier it was ok but there is slight decrease in period flow